Welding became a developed means of joining metals during World War I and has since expanded to become one of the most widely used joining techniques. It is commonly applied in applications in which leak-tightness is important, as in containers or conduits for liquids or gases, such as ships or pipelines. Welding generally involves local melting of the materials to be welded with or without the application of pressure; cold welding is also possible using high pressure rolling or hammering. The heat needed in hot-welding is typically generated using electric current as in resistance welding, including seam welding and spot welding, or in electric arc welding. Melting can also be accomplished with a gas flame as in fusion welding with an oxyacetylene torch.
The technique of using heat generated by friction has been found to produce a combination of metal deformation and heat sufficient to allow metal to be joined without actually melting. In particular, friction-stir welding (FSW) is a process disclosed by Thomas, et al. in U.S. Pat. No. 5,460,317 entitled "FRICTION WELDING" as a means of producing low cost, solid-state welds with low-distortion and reduced cracking in the heat affected zone of the joint. FSW allows the joining of alloys that are normally difficult to weld by processes which require melting, such as aluminum alloys 2090 (Al--Cu), 6082 (Al--Mg--Si), and 7075 (Al--Zn--Cu--Mg). In the FSW process, a small probe is set into rotation and translated along the length of the joint to be welded. The frictional heat generated as a result of the contact between the probe and workpiece locally heats and plasticizes the workpiece metal. This plasticized metal deforms and flows around the moving probe and cools as the rotating probe passes to leave a welded joint. FSW offers a number of advantages over other welding techniques as described by C. J. Dawes and W. M. Thomas in the article entitled "Friction-stir Process Welds Aluminum Alloys" which was published in the March, 1996, issue of The Welding Journal on pages 41 to 45. These advantages include the elimination of consumables such as cover-gas or electrodes, the elimination of toxic fumes, and the fact that metal welded with the FSW process is not liquified during the welding process.
FSW does, however, require that the metals being welded be firmly clamped. Also, as currently developed, the speed of welding is slower than and the welding equipment is not as portable as that for other welding processes.
The probe which is used to produce frictional heat must, of necessity, play an important role in friction-stir welding, Thomas, et al. in U.S. Pat. No. 5,460,317 mention that the probe which enters the workpiece tapers outwardly in a direction towards the workpiece. They disclose a probe having an elongate axis which intersects the joint region and extends substantially parallel with the sides of the workpiece defining the joint region and a probe which defines an elongate axis which extends in a direction transverse to a plane parallel with the joint region. The probe they disclose has a cross-section that is substantially circular and is composed of a material harder than the workpiece. However, Thomas, et al. do not disclose any details of probe surface morphology and, in particular, do not indicate the advantages that have now been discovered to be associated with a probe surface morphology that entails an enantiomorphic relationship between the upper and the lower portions of the probe itself. The word enantiomorphic is used here to refer to objects which are in a mirror image relationship one to the other, such as a left hand and a right hand.